1. Field of the Invention
This invention relates to a drum brake comprising a mechanical expander spreading a pair of brake shoes apart. More specifically, this invention relates to a brake cable mounting structure which enables remote operation of the mechanical expander.
2. Description of Related Art
One known brake cable mounting structure for the drum brake is disclosed in the Japanese Patent Application Unexamined Publication Number 6-337027 filed by this applicant. FIGS. 4-7 explain a drum brake device employing this kind of the brake cable mounting structure. As depicted in FIG. 4, a pair of brake shoes 110, 120 are moveably mounted on a back plate 100 with shoe holding devices 111, 121. Lower adjacent ends of the brake shoes 110, 120 abut against a supporting portion 201 of an almost L-shaped anchor 200 while upper adjacent ends thereof are connected via an adjuster 130. An upper shoe return spring 160 is extended between the upper adjacent ends of the brake shoes 10, 120 and a lower shoe return spring 160 is extended between the lower adjacent ends thereof, maintaining the abutment of the two brake shoes 110, 120 against the adjuster 130 and the anchor 200.
A mechanical expander 300 comprising a brake lever 320; a strut 330; and a lever pin 310, is positioned adjacent to the supporting portion 201 of the anchor 200 between the brake shoes 110, 120 (see FIG. 4). As depicted in FIG. 5, the brake lever 320 comprising two facing long plates has a notched groove 321 formed at the superimposing portion on the right side of the plates being functionally engaged with the right brake shoe 110. An arc-shaped groove 322 formed on the forked legs on the left side of the plates receives a cable end nipple 420 of a brake cable 400. The strut 330, integrally formed from a deformed piece of plate, comprises two facing plate portions connected on their upper edges by a bridge 332 and positioned between the brake shoes 110, 120. A notched groove 331 formed at the superimposing portion on the left side of the strut 330 is functionally engaged with the left brake shoe 120.
The brake lever 320 is inserted from the opposite side of the bridge 332 into a space formed between the facing plate portions of the strut 330, and upper right ends of the brake lever 320 is pivotally supported relative to the strut 330 with the lever pin 310 as depicted in FIG. 5.
As is evident from FIG. 5, clockwise rotation of the brake lever 320 with respect to the lever pin 310 is restricted when the brake lever 320 abuts against the bridge 332 of the strut 330. A guide pipe 500 and the brake cable 400, which act as a remote force transmitting member, pass through the back plate 100 to engage the cable end nipple 420 with the arc-shaped groove 322 as an input force portion of the brake lever 320. The guide pipe 500 depicted in FIGS. 5-7 is so designed that the overhanging portion 501, integrally formed on the intermediate portion thereof, contacts a back of an anchor seat 202 of the anchor 200. An upper end 502 of the guide pipe 500 penetrated through a hole on the anchor seat 202 and projected outwardly from the surface of the anchor seat 202 is widened in opposite directions; therefore, the guide pipe 500 is integrated with the anchor seat 202 of the anchor 200. The widened end 502 of the guide pipe 500 is designed to be partially widened toward each side of the brake shoes 110, 120. However, the widened end shape is not limited to the above described one and may be a widened shape in the direction as long as it can secure the thickness thereof in the direction to the supporting portion 201 of the anchor 200 without becoming an obstacle to components of the drum brake.
The brake cable 400 is comprised of an outer casing 430, an inner cable 410 and so on. A large diameter portion of a casing cap 431, fixed at an end of the outer casing 430, abuts against an outer opening end of the guide pipe 500. A small diameter portion of the casing cap 431 fits into a bore of the guide pipe 500. Then, the brake cable 400 is retained on the guide pipe 500 by a wire spring clip 440. A means to retain the casing cap 431 on the guide pipe 500 may be utilized until the operational end (not shown) of the brake cable 400 is attached on a corresponding member. As an alternative, the casing cap 431 may be press fit into the bore of the guide pipe 500 instead of using the spring clip 440.
The inner cable 410 is slidably inserted into the outer casing 430, and the topside thereof projected out from the casing cap 431 is passed through the guide pipe 500 as shown in FIG. 5. A pin portion of the cable end nipple 420 secured on the tip of the inner cable 410 is connected on the arc-shaped groove 322 of the brake lever 320. A dust boot 411 with bellows is positioned and connected between the casing cap 431 and the cable end nipple 420, performing a water proof function into the outer casing 430. To be lower in weight than the drum brake, a relatively lighter plate material forms the back plate 100. A back plate stiffener 105 is provided only at the 110 area from the central portion to the portion around the anchor 200 on the back plate 100 where required certain high strength and is substantially integrated with the back plate 100 such as by welding.
The back plate 100 and the stiffener 105 are fixed on the brake fixing part 150 (e.g., nonrotatable part on an axle member of the vehicle) having almost same outline of the mounting surface as the stiffener 101 by four fixing bolts 140, 140, 141, 141 and corresponding nuts (not shown in Figures). Each of the two bolts 140, 140 at the anchor 200 side has a serration 142 on its intermediate portion. The serrations 142, 142 are pre-press-forced into the back plate 100 and the back plate stiffener 105, thereby temporary fixing the anchor seat 202 of the anchor 200 and slidably supporting the right side of the brake lever 320 and the left side of the strut 330 on the vertexes of the fixing bolts 140, 140. Finally, the anchor 200 will be firmly fixed to the brake fixing part 150.
Brake operation of the above-explained structured device is explained below. If the operation side of the inner cable 410 (not shown in the figure) is pulled, the intermediate portion of the curved outer casing 430 tends to be deformed to a straight line shape. The casing cap 431 is supported by the guide pipe and the other side of the outer casing 430 (not shown in the figure) are supported by the corresponding member in order to prevent this deformation, thereby transmitting the pulling force onto the cable end nipple 420.
As the pulling force is transmitted to the arc-shaped groove 322 functioning as the input force portion of the brake lever 320, the brake lever 320 rotates counterclockwise in FIG. 5 with respect to the lever pin 310 to move the brake shoe 110 outward, and that reaction force urges the strut 330 to push the brake shoe 120 via the lever pin 310. If such a pressing force overcomes a tension of the shoe return springs 160, 160, both brake shoes 110, 120 spread apart at the point of abutment on the adjuster 130, thereby making a frictional engagement with the brake drum, not shown in the figure.
In FIG. 4, when the brake drum (not shown in the figure) rotates clockwise, the brake shoe 110 is abutted by the supporting portion 201 of the anchor 200, and the left brake shoe 120 becomes supported by the adjuster 130, thereby generating a braking force. Contrary, the brake drum rotates counterclockwise, the brake shoe 120 is abutted by the supporting portion 201 of the anchor 200, and the brake shoe 110 is abutted by the adjuster 130, thereby generating a braking force. Accordingly, both brake shoes 110, 120 have a self-servo function even if the brake drum rotates in either direction, i.e., functioning as a duo-servo (DS) type drum brake.
The above-conventional drum brake suffers from the following drawbacks and deficiencies.
The guide pipe 500 is designed so that the overhanging portion 501 contacts the back (the back plate 100 side) of the anchor seat 202 of the anchor 200, and the upper portion thereof is penetrated through the hole on the anchor seat 202. The known end portion 502 is widened at the terminal end of the guide pipe 500. With this structure, an effective stroke of the brake cable 400 is restricted by a gap between the lower end surface of the brake lever 320 and the widened end 502 of the guide pipe 500. If the distance (brake off-set) H from the brake mounting surface to the center of the brake shoes 110, 120 is too short, it becomes difficult to design the layout of the brake cable mounting section and of the mechanical expander 300.
The overall length of the guide pipe 500 must be longer, which is another disadvantage when considering the cost of manufacturing.
During the brake operation, the anchor 200 receives the brake force of the brake shoes 110, 120 and the operational reaction force on the outer casing 430 via the guide pipe 500. Therefore, the anchor 200 needs to be stronger which is another disadvantage in considering the weight and cost.
This invention improves upon the aforementioned problems in the prior art and provides a brake cable mounting structure for a drum brake, in which even if the distance from the brake mounting surface to the center of the brake shoes in the width direction is short, an efficient and effective layout of the brake cable mounting section and the mechanical expander is facilitated. Further, the load acting on the anchor may be reduced, which eliminates the uncertainty of the anchor strength.
With this invention, no projection on the anchor seat toward the mechanical expander side (i.e., widened end of the guide pipe in the conventional part) is necessary; therefore the range of the effective stroke of the brake cable is increased. Accordingly, even if the distance from the brake fixing portion to the brake shoe center in the width direction, i.e., brake off-set, is short, the layout of the brake mounting section and the mechanical expander is facilitated.
Further, an operational reaction force during the parking brake operation is supported by the back plate. Hence, the anchor supports the braking force, which enables to be designed smaller and lighter.
A widened end of the guide pipe is positioned in the pocket, thus, a through hole for the inner cable with the cable end nipple opened on the anchor seat may be made smaller which also reduces the size of the anchor.
Since the guide pipe is mounted on the back plate, the overall length of the guide pipe is reduced, which also reduces the cost of manufacturing the device.
The objectives and advantages of this invention are achieved by a brake cable mounting structure for a drum brake comprising a mechanical expander having an input force portion, where the expander is disposed adjacent to a pair of adjacent facing ends of brake shoes moveably mounted on a back plate so as to actuate said brake shoes. The mounting structure also comprises a brake cable having an inner cable is connected with said input force portion of the mechanical expander and the brake cable has an outer casing. The mounting structure further comprises a guide pipe guides the inner cable to pass out of said drum brake, where the outer casing is attached to said guide pipe, and where the guide pipe is fixed on said back plate.